1. Field of the Invention
The present invention relates to a technology for switching between two layer 2 switches as a destination of IP packets from a plurality of cards of a base station apparatus which transmit IP packets through either one of the two layer 2 switches. A layer 2 switch refers to a switch for delivering an IP packet depending on the data of the IP packet in a layer 2 (data link layer).
2. Description of the Related Art
Generally, redundant configurations include a dual active system configuration having two systems both used as active systems and an active-backup system configuration having two systems used as an active system and a backup system, respectively. Active-backup system configurations are disclosed in JP-A-1999-246646, JP-A-1993-344144, JP-A-1997-135244, and JP-A-2003-234757 (hereinafter referred to as Documents 1 through 4, respectively).
Dual active and active-backup schemes need to be used differently depending on the system architecture that is involved. For example, a terminating unit for terminating a transmission path is generally installed in each transmission path. If a system architecture has a plurality of such transmission paths, then each transmission path with a terminating unit installed therein is used as an active system, and two such active systems are used as a dual active system configuration. If the system architecture has only one transmission path, then a plurality of terminating units are combined with the transmission path, and only the terminating units are provided in a redundant configuration. One of the terminating units belongs to an active system, and the other terminating units to a backup system, resulting in an active-backup system. Even the same system architecture needs to be compatible with either of the dual active system and the active-backup system depending on the transmission path or paths.
Layer 2 switches usually forward IP packets received from cards in a system to corresponding cards. The layer 2 switches also forward IP packets received from transmission paths to corresponding cards. Therefore, layer 2 switches may be mounted in terminating units. The layer 2 switches need to be compatible with either of the dual active system and the active-backup system.
The technologies disclosed in Documents 1 through 4 referred to above are related to active-backup system configurations, but not dual active system configurations. If a layer 2 switch that operates in an active system suffers a fault, then each card is required to switch the destination of IP packets to a layer 2 switch in another active system or a backup system. Heretofore, however, a dedicated device such as a controller disclosed in Document 4 is necessary for switching between layer 2 switches as a destination of IP packets.
A layer 2 switch includes therein a buffer for holding IP packets. When the destination of IP packets is switched from a layer 2 switch to another layer 2 switch, IP packets in the past remain held in the layer 2 switch which was active prior to the switching, tending to result in a packet loss. Such a packet loss may be eliminated by sending at all times replicas of IP packets to the layer 2 switch to which the destination of IP packets is switched. However, this process is subject to the danger of a duplication of IP packets during the switching.
Generally, IP networks are not supposed to guarantee a sequence of IP packets. However, if a sequence of IP packets is not guaranteed, then a receiver of the IP packets needs to perform some process of correcting the sequence in which the IP packets are received. A closed network in an apparatus, e.g., an IP network constructed only within an apparatus, can simplify the reception process of each card by guaranteeing a sequence of IP packets. Closed IP networks in apparatus are thus desirous of having layer 2 switches of a redundant configuration which are capable of guaranteeing a sequence of IP packets.